Carbocyclizations five-membered metallacycle

Evans and co-workers simultaneously developed of the rhodium-catalyzed [4-+2 + 2] car-bocychzation reaction through the identification of a novel mode of reactivity for the five-membered metallacycle intermediate [14, 37]. Evans proposed that 1,3 dienes should be amenable to addition to this intermediate, which would provide a new reaction pathway. The tethered enyne 80 (Scheme 12.9), was expected to afford the metallacycle I, which, upon sequential incorporation of a butadiene and reductive ehmination, would afford the desired 5,8-fused product 81. Additionally it was rationahzed that this transformation should be highly diastereoselective, as outlined in Scheme 12.10. Treatment of the heteroatom-tethered enyne I under standard carbocyclization (Pauson-... 

Similarly to cyclopropanes, four-membered carbocyclic compounds undergo oxidative addition to low-valent transition metals to form five-membered metallacycles. Rhodium(I) inserts into C-C bonds next to the carbonyl group of ketones to form a rhodacycloalkanone species [49]. The C-C bond of cyclobutanone was cleaved, even at room temperature, by oxidative addition to a rhodium(I) complex having a PBP pincer ligand [50]. In the case of cyclobutanone 70, catalytic decarbonylation was possible and afforded the alkene 71 and cyclopropane 72 (Scheme 3.40). 

Construction of carbocycles from metallacycles is attractive. Theoretically, for example, reaction of a five-membered metallacycle with a one-carbon unit may afford a five-membered carbocycle. A six-membered carbocycle may be formed from insertion of a two-carbon unit into a five-membered metallacycle. However, in many cases, skeletal rearrangement of in situ generated metallacycles takes place, thus making prediction of structures of products very difficult. Accordingly, in this section, we focus on the carbocycles formed, regardless of the starting metallacycles. 

Conversion of the five-membered metallacycles into five-membered carbocycles, where the metal is eventually replaced with one carbon atom, is an attractive method for the construction of five-membered carbocycles (Eq. 38). In fact, such transformation is very popular using five-membered metallacycles of titanocenes and zirconocenes [1-5]. 

In conclusion, five-membered metallacycles of titanocene and zirconocene are convenient starting materials for the construction of five-membered car-bocyclic compounds. The formation of five-membered carbocycles can be accomplished by addition reactions (or insertion reactions) of a variety of electrophiles, such as CO, RCN, RNC, bis(trichloromethyl)carbonate, allenyl carbenoids, halogencarbenoids, aldehyde, acyl chlorides, propynoates, and iodopropenoatesthe to the five-membered metallacycles. 

Insertion of a ttvo-carbon unit into a five-membered metallacycle may afford formation of a six-membered carbocycle (Eq. 60), such as a benzene derivative via a formal [2+2+2] aromatization of three alkynes. Similarly, insertion of a C-X unit such as a nitrile into a five-membered metallacycle may afford formation of a six-membered heterocycle, such as a pyridine derivative (Eq. 60). In recent years, Takahashi laboratory and other laboratories have developed a number of synthetically useful methods for six-membered cyclic compounds by taking advantage of five-membered metallacycles of zirconocenes and ti-tanocenes [1-5]. 

VCPs have served as valuable five-carbon components in various cycloaddition reactions. Usually, they form six-membered metallacycles upon oxidative cycliza-tion with transition metals. Then, migratory insertion of unsaturated molecules, followed by reductive elimination, furnishes the carbocycles. In particular, intermolecular [5-1-2] annulation of VCPs with alkynes, alkenes, and allenes has been studied extensively (Scheme 2.39) [57]. In addition, VCP-cyclopentene rearrangement has been well documented [56 ]. 

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